At the present time, the major part of the zinc which is produced from ore is produced by hydrometallurgy, with the production of metal zinc by electrolysis of aqueous acid solutions of sulphate in vessels which are provided with insoluble anodes. The electrolysis baths contain free sulphuric acid and at the same time as zinc is deposited at the cathode, oxygen is given off and free sulphuric acid is formed at the anode.
The nature of the metal forming the insoluble anodes is selected on the basis of the following considerations: the anodes must be capable of resisting corrosion in a sulphuric acid medium and in the presence of nascent oxygen, and the polarisation voltage acquired by the anode must be low. In actual fact, in the production of a metal by electrolysis, the energy cost is a substantial part of the cost price, and the level of efficiency in regard to the consumption of energy in electrolytic reduction, which is partly determined by anode polarisation, cannot be neglected. The problems involved with insoluble anodes are frequently considered in regard to the electrolytic deposits of coatings using high-cost metals, where the energy costs are a less important part of the overall cost, while the qualities of the metal deposited are more important. In addition, the electrolytic production of metals is a heavy industry, so that the problems in regard to tonnages and handling are of real importance.
The limitations in regard to corrosion resistance and a low level of anodic polarisation have resulted in lead being virtually universally used as the anode metal. The lead contains from 0.25 to 1.0% by weight of silver which improves the mechanical qualities of the anodes (increase in rigidity and hardness), and also resistance to corrosion in the presence of impurities in the baths, in particular chlorides.
Lead anodes are generally plates of rectangular shape, with geometrical surface areas ranging from 0.55 to 1.7 square meters, the thicknesses of the plates ranging correlatively from about 8 to 16 mm, and the weights of the plates being from 50 to 300 kg. It should be noted that the anodic surface areas of the plates are double the geometrical surface areas, with both faces of the plate being active as an anode. To give an idea of the order of magnitude, an electrolysis shop producing 100,000 tonnes of zinc per annum uses 2376 tonnes of lead, containing close to 12 tonnes of silver, for the anodes, that is to say, close to 10,900 plates of a unit weight of 218 kg. In an installation of this type, the capital investment cost in respect of anodes may be up to 20% of the total investment. It is clear that a reduction in the weight of the anodes would have substantial repercussions on the capital investment and also on the costs involved in the handling operations (each anode is removed from the bath 6 to 8 times per year, which represents overall from 220 to 300 handling operations daily). However, the mechanical properties of the lead used do not permit the thickness of the plates to be reduced, without running the risk of deformation when the plates are being handled and in operation thereof, and premature deterioration.
It is possible to envisage using composite anodes with a stiffening reinforcing member of mechanically strong metal, with the reinforcing member being enclosed in a lead sheath. It is generally the usual practice in electro-chemical operations to use electrodes with an active surface which is adapted to the electrochemical use in point and which is plated to a core portion or reinforcement, the nature of which is so selected as to be appropriate to a particular situation (cost, compatibility with the active surface portion, facility of machining, mechanical strength, electrical conductivity, and the like).